This invention relates to transport systems such as those disclosed in copending application Ser. No. 545,339, filed Jan. 30, 1975, now U.S. Pat. No. 4,012,039, and in U.S. Pat. Nos. 3,724,691 of Apr. 3, 1973; 3,797,405 of Mar. 19, 1975; and 3,881,425 of May 6, 1975.
In the above systems, a wheel-supported vehicle or a train of them moves through a conduit system through which air is pumped. Each vehicle incompletely fills the cross section of the conduit system and presents a high coefficient of drag with respect to the air being pumped through the conduit system. The air flow is maintained at a high mass rate of flow and vehicles are swept along by this flow of air.
In the development of these systems, several types of air pumps were employed. One type, as disclosed in U.S. Pat. No. 3,797,405, may be regarded as a jet type of pump in which air is withdrawn from the conduit system and discharged back into the conduit system through what is effectively a restriction or nozzle, the resultant high velocity of the jet stream discharge being partially converted, downstream of the discharge region, into a pressure component which creates or maintains the requisite momentum for the high mass rate of flow. Although this type of pump system is low cost and reliable, the amount of air which can be withdrawn from the conduit system and then reintroduced thereinto under high velocity, is limited to a fraction of the total flow in the conduit system in order to avoid a "dead spot" or any tendency for reverse flow in the conduit system between the regions of air withdrawal and reintroduction by the pump. Consequently, in order to provide a system having high total energy input with this type of pump, a large number of them are required in series along the pipeline or conduit system.
Subsequent developments involved the use of pumps which develop the requisite air momentum not by a high velocity jet stream discharge but by directly discharging the high mass rate of flow into the conduit system, thereby creating a tendency for the air to flow reversely or in upstream direction from the region of introduction. The air inducted by the pump may either be withdrawn from the conduit system or it may be ambient air, depending upon whether the pump is employed as a booster along the conduit system or whether it operates as an end-of-line pump. In any event, some physical obstruction is required immediately upstream from the region of air introduction to prevent air reversal through the conduit. In U.S. Pat. No. 3,881,425, the vehicles themselves are used for this purpose and in order to move the vehicles to a point beyond or downstream of the region of air introduction or discharge in order to get them moving within the conduit system, a mechanical drive unit may be employed. Although this type of system is very efficient, it does require the complexity and disadvantage of a mechanical drive unit of some kind and also imposes a program of vehicle movement through the system.
In order to eliminate the constrants of the immediately foregoing systems, a further pump arrangement was developed, as disclosed in application Ser. No. 545,339. In such systems, the physical obstruction required to prevent any significant reverse flow of air is provided by an overhead-suspended, counterbalanced flapper valve which is responsive to the phenomenon of an approaching vehicle automatically to swing upwardly out of the way of the vehicle, allowing it to pass therebeyond and back into the mainstream of the moving air mass. The valve closes automatically behind the vehicle or train.
The present invention is directed to systems using two valves which allow still greater pressure differentials to be employed. While U.S. Pat. No. 1,753,987 describes a system using two valves for moving a postal capsule, such a system is unsatisfactory for moving wheeled vehicles.